Femtosecond laser offers high-speed DNA diagnostics

05 Jul 2006

Researchers in Canada have developed a new laser that, they say, enables high-throughput analysis of optical DNA biosensors.

A group of researchers at Toronto University has built a diode-pumped femtosecond extended cavity Yb:KGW laser that operates at a reduced repetition rate of 15 MHz. The team claim that the design, which exploits commercially available telecom components and off-the-shelf optics, allows more rapid characterization of DNA with optical biosensors (Optics Express 12 5286).

"Our laser is a simple, reliable and cost-effective source for applications in the fast-paced arena of optical DNA sensor technology development," Arkady Major, one of the lead researchers, told optics.org. "The system is a distinct and elegant alternative to other commonly used laser sources."

The researchers say that their laser enables high-speed measurement of fluorescence lifetimes, the amount of time a fluorescently-labelled piece of DNA emits light before emitting a photon. Fluorescent labels are used to gain insight into reactions between separate DNA strands, since changes in fluorescence lifetime indicate whether the DNA strand is in solution or bound to another piece of DNA.

Most measurements of fluorescent lifetimes exploit pulsed dye lasers with much lower repetition rates - in the 10-100 Hz range - and a pulse duration of a few nanoseconds. "Compared to conventional nanosecond-range pulsed dye lasers, our technique allows a more accurate and high-speed measurement of fluorescent lifetimes due to a much higher repetition rate of 15 MHz and a much shorter pulse duration of 200 fs," said Major.

According to Major, commercially available fiber lasers operating at a wavelength of 1 µm and a low-MHz repetition rate can also be used, but they rely on the more complex and expensive concept of chirped pulse amplification. "This requires the use of additional pump source for an amplifier, diffraction gratings for pulse stretching/compression and high-voltage electronics for pulse switching," added Major.

Laser pulses

To avoid aliasing and cumulative effects in measuring fluorescence decay lifetimes, the researchers used two identical optical telescopes to extend the cavity length by 8 m. This ensured that the period between laser pulses was 3-5 times longer than the time between successive excitations. By passively mode-locking the laser at a repetition rate of 15 MHz (67 ns), the researchers say that they can accurately measure label dyes with fluorescent lifetimes of up to 20 ns. What's more, the researchers say that the new laser is not limited to DNA diagnostics. "Owing to its 200 fs pulse duration and reasonable pulse energy, the laser is also useful for fluorescence lifetime imaging, two-photon excitation, and second- and third-harmonic generation microscopy," said Major.

Other analytical application areas include immunoassays for the detection of antibody-antigen interactions, and receptor-ligand interactions that are associated with drug discovery. "The longer fundamental wavelength of our laser also provides reduced scattering and larger penetration depth in biological tissue than commonly used Ti:Sapphire lasers emitting around 800 nm," concluded Major.